1. Field Of The Invention
The present invention relates to methods and apparatus for producing shaped, crosslinked, extruded polymeric products and more specifically relates to the production of shaped, crosslinked, extruded polyethylene products.
2. Prior Art
The preparation of graft copolymers of polyolefins such as polyethylene and hydrolyzable olefinically unsaturated silanes in the presence of a solvent, e.g., xylene, and using a peroxide or hydroperoxide catalyst is disclosed in U.S. Pat. No. 3,075,948.
The production of shaped crosslinked polyethylene is disclosed in U.S. Pat. No. 3,646,155 in which polyethylene and a hydrolyzable, olefinically unsaturated silane are first reacted in an extruder in the presence of a peroxide catalyst to form a graft copolymer which is extruded and granulated and mixed by tumbling with a blend of polyethylene and a silanol condensation catalyst made in a second extrusion. The resulting composition is extruded in a third extrusion to provide a crosslinkable composition. While it is possible to carry out the process of this patent with only two extrusions, it still involves a large amount of handling and an excessive usage of extruder time, higher operating costs and the risk of deterioration of the graft copolymer during storage. Other patents disclosing the need for similar multi-extrusions, i.e., two or more extrusions, in order to provide a crosslinkable product include: U.S. Pat. Nos. 3,802,913; 4,117,063; 4,136,132; and 4,228,255.
It is difficult, if not impossible, or impractical, to adequately mix the silane, peroxide and silanol condensation catalyst into polyethylene by injecting them into a conventional extruder and there results the formation of small gels throughout the polyethylene mass, apparently because of localized premature crosslinking in areas of high additive concentration resulting from inadequate mixing. A one-extrusion process for making a crosslinkable polyethylene is disclosed in U.S. Pat. Nos. 4,117,195 and 4,351,790, herein called the "one-step" patents. In the processes of these patents, polyethylene, hydrolyzable unsaturated silane, free-radical generator and a silanol condensation catalyst are fed into the hopper of barrel of a specially designed, complex and expensive extruder. Extruders usable in the one-step process are substantially longer at 30:1 length to diameter ratio which is substantially longer than the 20:1 or 24:1 length to diameter of the majority of conventional extruders now in use. Additionally, a special extruder screw design as disclosed in British Pat. No. 964,428 is needed in the one-step process. In this special screw design, the material to be extruded is forced over the flight of the screw from a rapidly-converging blind passage into a very slowly converging passage, followed by a slightly converging zone and thence through a metering zone of uniform cross-section.
Furthermore, suitable control of temperature in the extruder of the one-step process is necessary to prevent the high temperature in the metering zone (where grafting is caused to occur at relatively high temperatures of about 230.degree. C. more or less) from creeping into the other zones to produce higher temperatures in the other zones which normally operate at about 130.degree. C. The temperature (230.degree. C.) used in the metering (grafting) zone of the one-step process is considerably higher than the temperature used in the grafting process of the earlier patentees, e.g., 135.degree. C. to 140.degree. C. in U.S. Pat. No. 3,075,948 and 180.degree. C. to 200.degree. C. in U.S. Pat. No. 3,646,155. The one-step process, moreover, is not readily adaptable to conventional extruders currently operated on production lines and new extruders and associated equipment must be purchased and installed in order to carry out the one-step process or difficult and expensive changes must be made if conventional extruders are converted.
Mixers are often employed at the discharge ends of extruders for the purpose of insuring the uniformity of the material being extruded. U.S. Pat. Nos. 4,169,679 and 4,302,409 disclose mixer heads adapted to be attached to the discharge end of an existing extruder forwarding screw by means of screw threads or any other means. The mixer head is disposed in the extruder barrel between the screw and the die and turns with the screw to mix the compacted, molten polymer delivered to it by the forwarding screw. A fluid additive, such as a blowing agent, can be introduced through the barrel wall to the molten polymer at the upstream end of the mixer head. Neither of these patents disclose the addition of a hydrolyzable, olefinically unsaturated silane or a peroxide activator or a silanol condensation catalyst to the polymer nor do they disclose conducting a grafting reaction in the extruder as a means for producing a crosslinkable polymer composition.
U.S. Pat. Nos. 2,540,146 and 3,035,303 illustrate the use of mixing heads constructed on the downstream end of an extruder screw upstream from the extrusion die but fail to disclose or suggest the provision of a mixer head adapted to be attached at the downstream end of an existing extruder of conducting a grafting reaction to produce a crosslinkable polymer composition.
Mixer heads which can be adapted to be fitted onto existing extruders are disclosed in U.S. Pat. No. 4,419,014 and British Pat. No. 930,339 in which the mixing elements on the facing surfaces of the rotor and stator are cavities. The molten polymer is forced by the forwarding screw through the cavities which apply a shearing, mixing action on the polymer as the rotor turns. British Pat. No. 1,475,216 discloses a mixer head that can be adapted to be fitted onto existing extruders in which the mixer head utilizes grooves and lands on the facing surfaces of the rotor and stator. None of these patents disclose or suggest the possibility of carrying out the grafting reaction in the mixer head to produce a crosslinkable polymer composition.